The national police chief has asked for help from the prime minister in rooting out corrupt immigration police officers involved in high-profile crimes including the Erawan shrine bombing and human trafficking.
A key suspect in last month's deadly Bangkok blast paid a $600 bribe to illegally enter Thailand, police said on Thursday, highlighting widespread corruption at the kingdom's borders.
Adem Karadag, one of two foreigners arrested in connection with the Aug 17 attack, was detained at a flat in eastern Bangkok late last month. Police said he was found in possession of bomb-making paraphernalia and dozens of fake Turkish passports.
Mr Karadag's role has not been explained by police, who say they now know more about how he came to be in Thailand.
"The first man (Mr Karadag) said he travelled through Vietnam to a neighbouring country where he then paid for transportation," police chief Somyot Pumpunmuang told reporters, without specifying which neighbouring country.
"At the Thai border he paid $600 (to cross into Thailand)," he added, without revealing the identity of the official who took the bribe.
A second man, Yusufu Mieraili, was detained two days later on the border with Cambodia. He allegedly was found in possession of a Chinese passport which police believe is genuine.
Police say Mr Mieraili has confessed to delivering the backpack bomb to another man who left it at the Erawan shrine minutes before the explosion.
Thailand is a notorious sanctuary for on-the-run foreigners and visa overstayers, with officials often willing to take a bribe to turn a blind eye to illegality.
In recent days, Pol Gen Somyot, himself a former deputy commander with immigration police, has railed against the ease of buying off border officials.
"I cannot ignore this problem because I feel ashamed," he told reporters on Wednesday as he called on the junta to help him clamp down on corrupt border officials.
Although Pol Gen Somyot did not confirm which country Mr Karadag entered from, it is likely to be Laos or Cambodia, both of which sit between Thailand and Vietnam.
Since the bombing, at least six police officers have been removed from their posts bordering Cambodia where Mr Mieraili was apprehended after they reportedly took bribes to let people pass.
Thai media reported on Thursday Pol Gen Somyot had relayed to the prime minister a list of six corrupt practices commonly committed by immigration police.
A memorandum on "Report on behaviour of immigration policemen" dated Sept 9 was sent to the premier and Pol Gen Prawit Wongsuwon, deputy prime minister for security.
The memo outlines six common corrupt behaviours of the immigration police:
Extending stays unlawfully in exchange for money
Violating entry and exit protocols when agents are aware foreigners have no valid reasons for extending stay
Allowing or taking bribes for illegitimate entries by people with and without passports. For those with passports, information is not properly entered into the database. This facilitates human trafficking.
Putting fake entry stamps to enable people to go to a third country. Some officials steal the stamp to verify entry of a person to create a source country that can be used to request permission to travel to a third country
Selling the TM 6 departure card, especially among workers in Thailand, found mostly in northeastern passes
Charging extra fees for visas on arrival, mainly at Suvanarbhumi and Don Mueang airports
"Immigration is the first line of defence as it screens people entering and leaving the kingdom and plays a pivotal role in national security. To make the investigation as transparent and fair as possible, I would like to ask you to consider ordering agencies in charge of investigating public servants to take action in this case," Pol Gen Somyot wrote.
It was also reported Pol Gen Somyot had issued an urgent order for inspector-general Pol Lt Gen Nares Nontachote to conduct an inquiry into immigration police and report the findings to him by Sept 23.
Prime Minister Prayut Chan-o-cha has special powers under the interim constitution over other branches of government and can take any action without concern of legal consequences.